1. Field of the Invention
The present invention relates to a device for calibrating extruded lines from hollow chamber sheets formed of thermoplastic resin. The present invention also relates to hollow chamber sheets that have been manufactured on an extruder with the inventive calibration device.
2. Discussion of the Background
DE-C 32 44 953 and EP-B 158 951 describe vacuum mold channels which are used to calibrate hollow chamber profiles out of thermoplastic resin. These documents disclose that single piece extruded hollow chamber profiles are guided through a channel of two cooling plates that are equipped with vacuum channels. Sinking of the hollow chamber profile due to gravitational forces during the cooling phase is counteracted by a supporting force resulting from the difference in pressure between the interior and exterior of the hollow chamber profile.
DE 3 526 752 describes a procedure and an apparatus for the production of hollow chamber sheets out of resin. In this procedure, the upper and lower flanges are extruded and then bonded with prefabricated fins. The upper and lower flanges are taken in and cooled with the help of vacuum calibrators. This procedure offers the benefit of being able to achieve any fin design and combine various resins on the hollow chamber profiles.
DE-U 9 014 958 describes a procedure and an apparatus for the extrusion of hollow chamber sheets out of thermoplastic resin. In this procedure, the flanges and the fins of the profile are extruded separately and then welded to each other while they are still in their thermoplastic condition. This is done to prevent sink marks that can occur due to thermal contraction of the fins during the cooling phase, particularly in the case of single piece extrusion. The hollow chamber sheet profile obtained this way is immediately guided through a calibration device with an upper and a lower endless belt which preferably consists of metal. The cooling plates, which are located above or below the endless belts and through which coolant flows, serve to release heat. In order to maintain good contact between the endless belts and the cooling plates and to guide the endless belts at an established and even distance, the cooling plates can be equipped with vacuum channels via which the endless belts are taken in. This calibration method has the disadvantage that the hollow chamber profile is not supported by vacuum forces against gravity related sinking so that the method cannot be used on single piece extruded hollow chamber profiles.
Common vacuum calibration devices, where frictional forces occur between the extruded surface and the cooled metal plates of the calibration device (which also have vacuum openings), have a variety of disadvantages. Particularly on scratch-sensitive resins, the gliding of the extruding surface during calibration may cause the extruded surface to become scratched. Scratching of the extruded surface may lead to other problems if the abrasions accumulate.
The change between sticking and gliding (xe2x80x9cstick-slipxe2x80x9d) between the extrudate (i.e., the extruded surface) and the calibration device leads to an uneven draw of the extrudate. As a consequence of the uneven draw, fluctuations in the thickness of the extrudate in the extrusion direction may occur. These fluctuations may cause noticeable waviness on the hollow chamber profile. This waviness impairs the transparency of fin plates formed out of the transparent resin.
Cooling related to shrinkage of the fins when the fin plate runs through the calibration process (and also due to the pressure of the upper and lower flanges on the calibration surface) can lead to the formation of sink marks in the area of the fins. The sink marks become more distinct when the temperature difference of the fins between entering and exiting calibration increases. The sink marks also become more distinct if the negative pressure becomes lower and if the fins become thicker.
High negative pressure, which is desired for good thermal transmission between calibration and the strap surface and for avoiding sink marks, leads to high draw forces due to the frictional forces between the calibration device and the extrudate. In extreme cases, high draw forces can cause the extrudate to rupture between the calibration device and the drawing equipment.
The drawing rollers can also slip on the extrudate surface. This slipping may cause the extrusion process to collapse. In order to avoid such slipping, even at high negative pressure, complex multi-roller designs are required.
Too large a temperature difference can lead to the formation of internal stresses in the extrudate. Thus, the drawing speed that can be achieved with dry vacuum calibration is limited by the maximum allowable temperature difference between the temperature of the extrudate when exiting the extrusion die and temperature of the extrudate when exiting the calibration device. An extension of the calibration device as an alternative to increase the drawing speed is also problematic due to rising draw forces.
Accordingly, one object of the invention is to provide a novel and improved apparatus and method for the calibration of hollow chamber sheet extruded lines formed of thermoplastic resin.
Another object of the invention is to provide an apparatus and method for calibrating extruded lines in which frictional forces on the extruded line are minimized.
These and other objects are achieved according to the present invention by providing an apparatus for the calibration of hollow chamber sheet extruded lines. The apparatus includes a vacuum housing and a planar intake area at a first end of said vacuum housing. The planar intake area is configured to receive the extruded line. Support rollers within said vacuum housing are configured to support said extruded line. Said extruded line exits said vacuum housing through a planar outlet area located at a second end of said vacuum housing opposite said first end.